Blocking oscillator



April 1949- D K. R. WENDT 2,468,420

BLOCKING OSCILLATOR Filed Dec. 11, 1945 INVENTOR I BY Karl RUlencH- %Zw Patented Apr. 26, 1949 BLOCKING OSCILLATOR Karl Rinner Wendt, Hightstown, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application December 11, 1945, Serial No. 634,257

6 Claims.

The present invention relates to oscillators, and more particularly relates to means for increasing the stability thereof.

Certain types of vacuum tube oscillators, commonly known as blocking oscillators, are so arranged that a condenser or other energy-storage element connected in the grid circuit of the tube is charged negatively during a portion of each cycle of oscillation. This negative charge attains, at one point in the cycle, a value suflicient to cut off or block the tube, which then remains blocked until the negative charge on the condenser has leaked off sufliciently to permit the tube to resume conducting.

The means for permitting the negative charge developed on the grid condenser to leak on in oscillator circuits of the above type usually consists of a relatively high resistor connected between ground and that plate of the condenser receiving the negative charge. By varying the value of this resistor, the rate of condenser charge dissipation, and hence the frequency of oscillation of the system, may be changed.

It has been found that the rate of grid-condenser discharge may be made more nearly linear if the negative plate of the grid condenser, instead of being connected through the grid resistor to ground, is instead connected through the grid resistor to a source of positive potential, such as that which furnishes the plate voltage for the oscillator tube.

Such an expedient, however, decreases to some degree the stability of the oscillator. In many applications, as where the oscillator forms part of a television transmitting or receiving system, frequency instability can not be tolerated. Hence the higher degree of waveform linearity obtainable with the grid condenser connected to a source of positive potential must be dispensed with. This is especially true in systems where the available source of positive potential is subject to random fluctuations, such, for example, as voltage dips resulting from load variations, which occur even when complex regulatory devices are employed.

According to a feature of the present invention, the frequency of an oscillator of the blocking type having its grid leak connected to a source of positive potential may be stabilized by the use of a fixed voltage in series with the grid resistor. This fixed voltage may take the form of a glow discharge tube. Such an arrangement causes the positive potential at the condenser terminal of the grid resistor to change at a more rapid rate than the fluctuations in the positive potential source. Hence, compensation may be brought about, and, by varying series and parallel impedances in connection with the fixed voltage, the amount of this compensation may be varied at will.

One object of the present invention, therefore, is to provide an improved form of blocking oscillator.

Another object of the present invention isto increase the frequency stability of a blocking oscillator of the type in which the grid condenser is connected through a resistor to a source of positive potential.

A further object of the invention is to provide, in a blocking oscillator of the type in which the grid condenser is connected through a resistor to a source of positive potential, a fixed voltage in series with the grid resistor so as to compensate for fluctuations in the value oi the positive potential source.

Other objects and advantages will be apparent from the following description of a preferred form of the invention and from the drawing, in which:

Fig. 1 is a circuit diagram of a blocking oscillator incorporating the present invention; and

Figs. 2, 3 and 4 are modifications of Fig. 1.

In Fig. 1 of the drawing is shown an oscillator including a high-vacuum tube l0 having an anode l2, a cathode i4, and a control electrode [6. A transformer 18 has one end of its primary winding Ell connected to the anode l2 of tube ill, and one end of its secondary winding 22 connected to the control electrode lb. The other end of primary winding 20 is connected to the terminal 2d of a source of positive potential, while the other end of secondary winding 22 is connected to one plate of a condenser 26. The transformer I8 thus produces an inductive coupling between the plate and grid circuits of tube It.

The upper plate of condenser 25, or, in other words, that plate of condenser 26 which is connected to the control electrode It of tube l0 through primary Winding 22, is joined to the terminal 2 1 of the positive potential source through an adjustable resistor 28, a glow tube 30, and a second resistor 32, all connected in series in the order named. An additional adjustable resistor 34 is shunted across the glow tube 30, while a still further resistor 33 joins the lower plate (in the drawing) of grid condenser 26 to ground. The elements 28, 3t, 32 and 34 provide a leakage path to the positive potential source 24 for the negative chargestored in the condenser 26, as will hereinafter more fully appear.

With the exception of the fact that the gridconnected plate of condenser 25 is grounded through adjustable resistor 28, instead of being connected to a source of positive potential through resistors 28 and 32 and glow tube 36, the illustrated oscillator is similar in many respects to that shown in U. S. Patent No. 2,101,520, issued December 7, 1937, to W. A. Tolson and J. R. Duncan. Since a complete description of the operation of the oscillator circuit is set forth in that patent, it will not be repeated herein, except to say that at the start of an operating cycle the plate current of tube it increases. Transformer i8 is so wound that at the instant the right-hand end (in the drawing) of the primary winding 20 is negative (for example), the right-hand end of the secondary winding 22 is positive. Hence an increase in plate current or tube l drives the grid or control electrode l6 positive, and this action is eifective to increase the plate current of tube Ill still further until a condition of saturation is reached.

The plate current of tube it then decreases at a rate and for a time determined in part by the natural period of oscillation of the plate and grid windings of transformer 18. The rate of current decrease is very rapid over a part of the cycle, however, since at the instant the plate current begins to decrease at a particular point in the cycle, the polarity of the grid i6 is reversed by the action of the transformer windings 2i and 22 with respect to each other. This drives the grid l6 rapidly to a negative potential far below that necessary for out off of tube i ii.

The negative potential thus produced on the grid of tube Ill is transient in nature, and the grid voltage accordingly returns to a value corresponding to the negative charge stored in the condenser 25 during the initial portion of the oscillation cycle, or, in other Words, during that portion of the cycle when the grid it draws current. The difference between this negative charge stored in condenser 26 and the cut-01f potential of tube I0 is the amount by which the negative charge on condenser 25 must be reduced before the plate l2 can again begin to draw current to start the next cycle of operation.

The time required for such a reduction in charge on condenser 26 is governed by the impedance of the discharge path between condenser 26 and the terminal 24 of the positive potential source. The value of this discharge path impedance is preferably chosen, by means of an adjustment of resistors 28 and (it, so that at the instant when conduction of tube ill should normally be initiated, the grid I6 is still slightly below cut-off potential. Synchronizing pulses of proper polarity may then be applied to the oscillator at terminal 38, these pulses being effective to instantaneously raise the potential on the grid at least to cut-off, whereupon conditions will again be such that the anode 52 can draw sufficient current to initiate the next succeeding cycle of oscillation. If desired, instead of synchronizing pulses being applied to the ocillator at terminal 38, AFC (automatic frequency control) voltages may be applied at terminal 50 from the plate of a D.-C. connected amplifier tube (not shown), or else any other suitable means for synchronizing the oscillator may be employed.

The voltage appearing across condenser 26 may have a substantially sawtooth waveform similar to that represented by the reference character 12.

This voltage, which may be used as the output voltage of the oscillator, may be taken off through a condenser ill having one plate connected to an output terminal 53 and the other plate to a point between condenser 28 and transformer winding 22. The output terminal &8, however, should not be loaded by a Value of resistance comparable to or smaller than the combination of resistors 28 and 32.

The linearity of the slowly rising portion of waveform d2 may be improved over that obtainable in the circuit of the Tolson et a1. patent, above referred to, by connecting the upper plate (in the drawing) of condenser 2i through the grid impedance to the terminal 2d of the positive potential source rather than to ground. How ever, as above stated, this mode of connection decreases the frequency stability of the oscillator.

According to a feature of the present invention, an oscillator of the blocking type with its grid resistor returned to a source of positive potential, such, for example, as the oscillator tube plate supply, may be frequency stabilized against random fluctuations in the output of the potential source by the use of a source of fixed voltage in series with the grid resistor. In the drawing this source of fixed voltage is represented by the glow tube 3@ across which there is a fixed Voltage drop. The effect of this source of fixed voltage is to compensate for changes in the value of the potential source connected to the terminal 24. For example, assume that for some reason the potential of the supply source connected to the terminal 2% decreases. Normally, this would cause an increase in the free-running frequency of the oscillator, due mainly to the change in the oscillator plate voltage. However, as a result of the action of the glow tube 363, the change in potential at the condenser end of resistor 28 is larger in proportion than the change in the potential of the supply source it, and is sufiicient to compensate for the change in the potential of the supply source. Hence the frequency of operation of the oscillator will remain substantially constant.

An output terminal which may be used as an alternative to the terminal to, if a low-impedance output connection is desired, may be obtained by effectively dividing condenser 26 by inserting a condenser 5i! of relatively greater capacity in series with condenser 26, as shown in Fig. 2. Point 52, between condensers 2s and 5t, will then have both lower impedance and less voltage than terminal 2 8 in Fig. 1, both of which conditions are desirable in many circuit applications.

The amount or" compensation provided by the glow tube 3t may be varied by adjustment of the series and parallel resistors 2c and 34 respectively. That is, the relatively fixed voltage drop across tube 39 may be changed by causing large variations in the current through it. Resistor 34 shunts the current through tube 30, and by adjusting this resistor, the current through tube 3!] may be varied over a considerable range. Resistor 28 is then adjusted to bring the oscillator back to its original frequency. The magnitude of the variation in the relatively fixed voltage obtainable in the above manner is dependent in part on the type of glow tube employed. If desired, additional glow tubes may be connected in series with tube 39 to permit larger variations.

It may be noted that normal voltage fluctuations cause a smallchange in the voltage across tube 30. This is due to the glow tube appearing electrically as a, fixedvoltage in series with an 7 internal resistance. Such internal resistance is unobjectionable, since the tube must be operated in series-with a resistance,-and the said internal resi'stance'may lee-considered as a portion of the necessary -'-totalcircuit resistance.

It should alsobe'noted-that the means of the present invention provides correction for changes in frequency of oscillation Y of the oscillator tube Ill due to variations in heater current flow through the tube heater 54. However, since there is an appreciable time lag between changes in heater current and corresponding changes in the oscillator frequency, it is desirable to make a compromise adjustment of resistors 28 and 34 so as to somewhat overcompensate for changes in the supply source 24 and to undercompensate for heater variations. In this way, it is possible to avoid excessive overcompensation of the system during the period between the instant of heater current and supply voltage change and the time when the heater current change is effective to vary the frequency of oscillation of tube II].

It should be understood that while complete compensation for both supply voltage and heater current changes is not provided for by the system of the present invention, nevertheless, such complete compensation is not usually necessary in practice. The two most troublesome sources of frequency change in oscillator circuits of the type herein described are sudden line voltage dips caused by the starting of some motor, for instance, and changes in plus B or supply voltage caused by necessary gain changes in other portion of the system in which the oscillator circuit is a component part. Neither of these sources of frequency change involve significant heater current variations.

Figs. 3 and 4 show modifications of the circuit of Fig. 1. In both of these circuits the resistors 32 and 34 have been omitted for simplicity of illustration, as well as the output capacitor M. Since the principle of operation of each of the circuits of Figs. 3 and 4 is similar to that of Fig. 1, it will not be set forth again. However, it should be noted that in Fig. 3, the condenser 26 discharges through resistor 28 and glow tube 36 to ground. The anode I2 of tube H] is at D.C. ground potential, while the cathode M of tube I0 is maintained at negative potential with respect to the anode i2 due to its connection to the negative terminal of the battery 56, the positive terminal of this battery being grounded as shown.

In Fig. 4 the condenser 26 is connected between the control electrode 16 of tube Ill and the winding 22 of transformer I8, rather than to the other end of this winding 22 as in Fig. 1. The negatively-charged plate of condenser 26 is joined to the supply source terminal 24 through resistor 28 and glow tube 30 in the same manner as in Fig. 1. As stated above, the mode of operation of the circuits of Figs. 2 and 3 is substantially the same in each case as that of the circuit of Fig. 1.

The following values have been found suitable for certain of the components used in the system illustrated in Fig. 1. However, it should be clearly understood that they are being given merely as an example, and that other values may be substituted therefor as may prove desirable or necessary:

Condenser 26:.05 mfd. Resistor 28=200,000 ohms Resistor 32 =150,000 ohms Resistor 34'=100,000 ohms Resistor 36:500 ohms Glow tube 30: 4 watt neon Having thus described my'invention, I claimf 1. Inan oscillator of Ithe type including agridcontrolled electron discharge tube,'a source of direct current potential which is subject to random fluctuations, a transformer having a pair of magnetically coupled windings, one winding connected between the positive terminal of said source of potential and the anodeofs-aidelectron discharge tube, the cathode of said electron discharge tube and the negative terminal of said source of potential being grounded, a condenser having one plate connected through the other winding of said transformer to the grid of said electron discharge tube, a first resistor, means connecting the other plate of said condenser through said first resistor to ground, and a second resistor connected between said one plate of said condenser and the positive terminal of said source of potential. the improvement which comprises a source of relatively constant voltage connected in series with said second resistor between the positive terminal of said source of potential and said one plate of said condenser.

2. An oscillator according to claim 1, in which said source of relatively constant voltage is a glow tube.

3. An oscillator according to claim 1, further comprising means for varying the value of said second resistor.

4. An oscillator according to claim 1, further comprising an impedance element in parallel relation with said source of relatively constant voltage.

5. In an oscillator of the type including a gridcontrolled electron discharge device having an anode and a cathode, a direct current power supply source which is subject to random fluctuations, the negative terminal of said power supply source being connected to the cathode of said electron discharge device and the positive terminal of said power supply source being connected to ground, a transformer having a pair of magnetically coupled windings, one winding connected between the anode of said electron discharge device and ground, a condenser having one plate connected through the other winding of said transformer to the grid of said electron discharge device, and the other plate joined to the cathode of said electron discharge device, and a resistor connected between said one plate of said condenser and ground, the improvement which comprises a source of relatively constant voltage connected in series with said resistor between ground and said one plate of said condenser.

6. In an oscillator of the type including a gridcontrolled electron discharge device having an anode and a cathode, a direct current power supply source which is subject to random fluctuations, a transformer having a pair of magnetically coupled windings, one winding connected between the positive terminal of said power supply source and the anode of said electron discharge device, the cathode of said electron discharge device and the negative terminal of said power supply source being grounded, a condenser having one plate connected to the grid of said electron discharge device and the other plate connected to the cathode of said electron discharge device through the other winding of said transformer, and a resistor connected between said one plate of said condenser and the positive terminal of said power supply source, the improvement which comprises a source of relatively constant voltage connected in series 7 8 with said resistor between the posit ive terminal UNITED STATES PATENTS of sand power supply source and send one plate 7 of said condenser. Number Name Date v KARL N WENDT 2,165,815 Rhea, July 11, 1939 5 2,227,075 Geiger Dec. 31, 1940 REFERENCES CITED 2,250,706 Geiger July 29, 1 1 The following references are of record in the gsg g fig file of this patent: 

